The present invention relates to electrolytic cells comprising polymeric film composition electrodes and separator membranes and to a manner of using such cells to provide highly efficient and economical batteries. In particular, the invention relates to unitary rechargeable lithium battery cells comprising an intermediate separator element containing an electrolyte solution through which lithium ions from a source electrode material move between cell electrodes during the charge/discharge cycles of the cell.
The invention is particularly useful for making such cells in which the ion source electrode is a material, such as a transition metal oxide, capable of intercalating lithium ions, and where an electrode separator membrane comprises a polymeric matrix made ionically conductive by the incorporation of an organic solution of a dissociable lithium salt which provides ionic mobility. More specifically, the present invention relates to the use of a class of polymeric compounds, in particular poly(vinylidene fluoride-co-chlorotrifluoroethylene) copolymers, in preparing such separator membranes and associated polymeric matrix cell electrode compositions.
Rechargeable lithium-ion battery cells, such as described in the incorporated disclosures, have generally been constructed by means of the lamination of electrode and separator/electrolyte cell elements which are individually prepared, by coating, extrusion, or otherwise, from compositions comprising polymeric materials, e.g., a plasticized polyvinylidene fluoride (PVdF) copolymer with hexafluoropropylene. For example, in the construction of a lithium-ion battery, a current collector layer of aluminum foil or grid was overlaid with a positive electrode film or membrane separately prepared as a coated layer of a dispersion of lithium intercalated electrode composition, e.g., a LiMn.sub.2 O.sub.4 powder in a copolymer matrix solution, which was dried to form the membrane. A separator/electrolyte membrane formed as a dried coating of a composition comprising a solution of the copolymer and a compatible plasticizer was then overlaid upon the positive electrode film. A negative electrode membrane formed as a dried dispersion coating of a low-voltage lithium insertion compound or intercalation material, e.g., a powdered carbon, in a copolymer matrix was similarly overlaid upon the separator membrane layer, and a copper collector foil or grid was laid upon the negative electrode layer to complete a cell assembly. This assembly was then heated under pressure, preferably in staged operations, to effect heat-fused bonding between the plasticized copolymer matrix components and to the collector grids to thereby achieve lamination of the cell elements into a unitary flexible battery cell structure.
The resulting laminated battery structure, which comprised a significant measure of homogeneously distributed organic plasticizer, particularly in the separator membrane stratum, was devoid of hygroscopic electrolye salt and, as a result, could be stored at ambient conditions, either before or after being shaped or further processed, without concern for electrolyte deterioration due to reaction with atmospheric moisture. When it was desired to activate a battery in the final stage of manufacture, the laminate cell structure was immersed in or otherwise contacted with an electrolyte salt solution which imbibed into the copolymer matrix to provide substantially the same ionic conductivity enhancement as achieved by a preformed hybrid separator/electrolyte film containing such an electrolyte salt solution.
In order to facilitate the absorption of electrolyte solution during activation, it is generally preferred that a substantial portion of the plasticizer be previously removed from the copolymer matrix. This may readily be accomplished at any time following the laminating operation by immersion of the cell laminate in a copolymer-inert solvent, such as diethyl ether, methanol, or hexane, which selectively extracts the plasticizer without significantly affecting the copolymer matrix of the cell element strata. The extracting solvent may then simply be evaporated to yield a dry, inactive battery cell which will readily absorb an effective amount of electrolyte solution that essentially replaces the extracted plasticizer.
The preferred poly(vinylidene fluoride-co-hexafluoropropylene)(VdF:HFP) copolymer materials described in the incorporated disclosures were selected from numerous candidate polymers on the basis of their unique balance of strength, flexibility, crystallinity, and solubility which were essential for the preparation of practical electrolytic cells. Such a combination of properties was determined to be lacking in most other polymeric materials, even such closely related terpolymers and copolymers as poly(vinylidene fluoride-cotetrafluoroethylene). It was thus surprising, indeed, when, in the instant invention, it was discovered that electrolytic cells and resulting rechargeable lithium-ion batteries of exceptional quality could be fabricated with the present poly(vinylidene fluoride-co-chlorotrifluoroethylene) (VdF:CTFE) copolymers.